1. Field of the Invention
This invention relates to a charging device which rapidly charges a secondary battery and more particularly to a charging device appropriate for charging a nickel-hydrogen battery.
2. Description of the Related Art
FIG. 1 shows a general charging curve showing the relationship between terminal voltage V and time T while a nickel-hydrogen battery is being charged, wherein P is a peak in the voltage; It is desirable to stop the charging at the time.
A conventional charging device of a secondary battery measures the terminal voltage of the secondary battery at given time intervals, and when it becomes equal to the immediately preceding measurement value, judges the voltage to be the peak of the charging curve and terminates the charging. Another conventional charging device is designed to stop charging when the voltage drops by a predetermined value from the past peak in the voltage, that is, when -.DELTA. V is detected.
The charging device of a storage battery disclosed in Japanese Patent Publication No. Sho 62-23528 calculates a slope amount of a charging curve sequentially and compares it with its adjacent slope amount for detecting in sequence an inflection point showing a point in the time when the slope amount changes from decrease to increase or from increase to decrease. When the inflection point matches a plurality of inflection points occurring in a predetermined order previously stored, the charging is stopped. That is, the charging device detects the boundary between regions II and III in FIG. 1, A, and that between regions III and IV, B. and detects inflection point B, then stops the charging.
As a further example, the charging device of a battery disclosed in Japanese Patent Publication No. Sho 53-40695 detects a slope of a charging curve and when the slope becomes equal to or greater than a predetermined value, stops the charging. That is, when detecting region III or IV in FIG. 1, the charging device stops the charging.
However, the region II in FIG. 1 may be formed extremely like a horizontal line depending on variations in the characteristic of batteries and a difference in ambient temperatures. In such a case, if the charging device is designed to judge it to be a peak if the immediately preceding terminal voltage equals the most recent terminal voltage, the peak is detected in the region II far before the real peak P.
If a device with a battery is used while the battery is being charged and when the terminal voltage temporarily drops, a dent occurs in the charging curve as shown in FIG. 2, and in its neighborhood, a peak of the curve or predetermined voltage drop -.DELTA. V or an inflection point is detected.
As compared with a nickel-cadmium battery, the nickel-hydrogen battery has a characteristic in which voltage drop -.DELTA. V is hard to appear. Further, a battery generates heat at full charging; the nickel-hydrogen battery has poor heat resistance and if heat is generated too much, performance of the battery degrades, thus a peak must be detected accurately.